Yes, what they dont mention that it will work in 2G emulation mode only, without 3G support. Omnia will even see 3G coverage, but you will not be able to make calls using WCDMA, it will just abort it on call, so to make it work in 3G-covered areas you'll have to disable WCDMA band in phone settings.
It should not matter what edition is it, Black, Silver or Gold, they only have minor differences in features (like voiceip autodial etc).
There are no dualSIMs on the market that support 3G SIM app protocol, if I remember correctly, because its much more secure.
The sim card from 3 will not work with any dual sim, because they have no 2G emulation.